1/15/12Gala[ieV and Whe UniYeUVe - HoW GaV in ClXVWeUV2/6ZZZ.aVWU.Xa.edX/keel/gala[ieV/icm.hWmlThe X-ray spectra reveal lines of highly ionized Fe, indicating that the gas has been processed - it came from galaxies earlyon, rather than being primordial. The galaxy motions in the cluster are an ample (but perhaps not the only) heating source,since kTa mpY2. A typical velocity dispersion 700 km/s implies Ta 6 x 107K from this source alone. Extra heating sourcesmay be sought from the correlation between velocity dispersion and ICM temperature, or considering the specific entropy ofthe gas.The hot gas has a very long cooling time (of order the Hubble time) except in denser areas, which can produce a coolingfloZonto individual galaxies or in cluster cores. This happens because of the interesting behavior of the cooling rate forhydrogen-dominated plasmas as a function of temperature and density; since brehmsstrahlung is important, the ne2dependence can pull the rug out from under a pressure-supported gas. That is, for a purely cooling plasma, the time it spends

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